1. Field of the Invention
This invention relates to a high quality Group III nitride semiconductor substrate.
2. Description of the Related Art
Recently, GaN based compound semiconductors such as gallium nitride (GaN), indium gallium nitride (InGaN), gallium aluminum nitride (AlGaN) are highlighted as materials for a blue light emitting diode (LED) and a laser diode (LD). Since the nitride semiconductor is excellent in heat resistance and environment resistance, the application of the nitride semiconductor to electronic device elements has been started.
At present, a sapphire substrate is practically and widely used as a substrate to grow the nitride semiconductor thereon, where the nitride semiconductor is in general epitaxially grown on the single crystal sapphire substrate by MOPVE (metalorganic vapor phase epitaxy) etc.
However, since the sapphire substrate has a different lattice constant from that of GaN, it is impossible to obtain a single crystal film by growing the nitride semiconductor directly on the sapphire substrate.
Further, since the MOVPE method needs a high temperature to cause a gas phase reaction, when temperature lowers after a GaN single crystal is epitaxially grown, a defect such as a warpage may be caused in the GaN due to the difference in thermal expansion coefficient between the sapphire substrate and the GaN.
JP-A-2003-37288 discloses a method that a buffer layer (“low temperature growth buffer layer”) of AlN or GaN is formed on the sapphire substrate at a low temperature to relax a lattice strain therebetween, and GaN is grown on the low temperature growth buffer layer. By using the low temperature growth buffer layer, the single crystal GaN can be epitaxially grown.
However, even in the method, since the lattice mismatch between the sapphire substrate and the epitaxial growth crystal cannot be eliminated, the epitaxial GaN layer includes a number of defects. Thus, the defect becomes an obstacle to producing a GaN-based LD and a high-brightness LED.
For the above reasons, a nitride semiconductor free-standing substrate is highly desired. In case of GaN, a large ingot thereof is difficult to be grown from a melt thereof unlike in case of Si or GaAs. Therefore, various methods such as a ultrahigh temperature and pressure method, a flux method and a hydride vapor phase epitaxy (HVPE) method have been tried. Of the above methods, the HVPE method is often used to develop the GaN substrate and the substrate thus developed has been distributed in the market, although gradually. Thus, it is expected that the GaN substrate is used for a high-brightness LED and a power conversion device as well as an LD.
In the Group III nitride semiconductor device where a higher output type is expected to be developed hereafter, it is very important to suppress a vacancy defect concentration as well as a dislocation density. This is because the vacancy defect may form a complex together with the other impurity such as oxygen to exert a harmful influence on thermodynamic and optical properties of the crystal.
Although the Group III nitride semiconductor has been improved in crystalline quality by various enhancing technologies, there is still a large difference in crystalline quality between the Group III nitride semiconductor and conventional semiconductors such as Si and GaAs.
It is assumed that the Group III nitride semiconductor may have a vacancy defect concentration more than the conventional high-quality semiconductors. Since the vacancy can exist in the crystal at a certain thermodynamic equilibrium concentration, especially in case of a high-output device which is subjected to a high temperature rise in operation, there is a concern that it causes an increase in vacancy concentration more than the conventional device.
In order to reduce the vacancy defect concentration, it is necessary to rapidly measure how it changes depending on the growth method and growth condition of the crystal. The positron annihilation is known as a method for estimating the vacancy defect concentration in the crystal.
The positron annihilation can estimate the vacancy defect concentration by using a phenomenon that positrons injected into a crystal have a longer lifetime at a vacancy position where the existing probability of electron is lower than that at a normal portion.
However, the positron annihilation is not an easy-to-conduct evaluation method since a special device is required for the positron annihilation experiment. Further, since the positron annihilation is an indirect measurement method, it is not always sufficient in measurement accuracy.
Further, since the positron annihilation is an evaluation method based on a very local measurement, it is difficult to make evaluation in a large area as provided for actual crystal growth. Therefore, any research has been seldom tried which is intended to reduce the vacancy defect concentration.